Although the researchers conceded that the emotional state of the bonobo might be to blame, the sound for "yes" stayed the same, even when Kanzi was in different emotional states.

Emotional state

Dr Taglialatela said: "We haven't taught him this - he's doing it on his own.

"That emotion is involved doesn't rule out that he's following rules that have some sort of cognitive component."

The definition of what actually constitutes "language" is controversial.

Some linguists believe that even symbolic communication - which many chimps achieve - qualifies as language, but many now say that some mastery of syntax is also required.

Primatologist John Mitani of the University of Michigan in Ann Arbor said: "Despite the fact that we have had glimmerings of this in the monkey world, few instances of anything like this have been documented among our closest living relatives, chimps and bonobos.

"There have to be evolutionary precursors to what we do. We are beginning to find them in the primate world."